FIG. 1 is a perspective view of a prior art semiconductor laser described in Electronic Letters, volume 23, 1987, pages 546-547. A schematic sectional view of the laser is shown in FIG. 2 to illustrate current flow in the laser. The illustrated structure includes a p-type InP substrate 1, a p-type InP lower cladding layer 2 disposed on part of the substrate 1, an InGaAsP active layer 3 disposed on part of the lower cladding layer 2, and an n-type InP upper cladding layer 4 disposed on the active layer 3. The lower and upper cladding layers and the active layer have a ridge structure that is confined at its longitudinal sides by a current confinement structure. The lower and upper cladding layers and active layer are grown by liquid phase epitaxy (LPE) and the ridge is formed by etching those layers with a mask present on what becomes the top surface of the ridge.
The current confinement structure illustrated in FIG. 1 includes a p-type InP layer 5 disposed on part of the substrate 1 and extending up the sides of the ridge to the upper cladding layer 4. An n-type InP current blocking layer 6 is disposed on parts of the p-type InP layer 5. An additional p-type InP current blocking layer 7 is disposed on the n-type layer 6 and part of the p-type layer 5. A second n-type upper cladding layer 8 is disposed on the p-type layer 7 as well as on the top of the ridge, i.e., in contact with the upper cladding layer 4. Finally, an n-type InGaAsP contacting layer 9 is disposed on the second upper cladding layer 8. As discussed in more detail below, the pn junctions within the structure constrain current to flow through the active layer in order to provide laser oscillation and light output. In order to further confine current to the ridge within the structure, an SiO.sub.2 film 20 including a window opposite the ridge is disposed on the top and side surfaces of the laser structure. A first electrode 21 is disposed on the substrate 1 and a second electrode 22 is disposed on the SiO.sub.2 film 20, contacting the InGaAsP contacting layer 9 through the window in that SiO.sub.2 film 20.
When a forward bias is applied across the laser of FIG. 1, electrons and holes are injected into the active layer 3 and recombine to produce light having an energy approximately equal to the band gap energy of the active layer 3. The light thus generated travels along a waveguide defined by the current confinement structure, the upper and lower cladding layers, and opposed facets transverse to the active layer 3. When the amplification rate exceeds a threshold, i.e., when the current flowing in the forward biased laser exceeds a threshold current, laser oscillation occurs and coherent light is emitted through one of the facets that is covered by a partially reflective coating.
As illustrated in FIG. 2, most of the current flows along the path indicated by C.sub.1, through the upper and lower cladding layers and the active layer. However, some leakage current flows along the paths indicated as C.sub.2 and C.sub.3. Path C.sub.2 includes a single pn junction whereas path C.sub.3 includes current flow through a pnpn structure. These leakage currents do not contribute to the light output of the semiconductor laser and increase as the current through the laser increases, i.e., as the voltage applied across the laser increases. The leakage currents elevate the temperature of the semiconductor laser and limit its power output. Even the pnpn thyristor current confinement structure does not adequately reduce the leakage current so that the power output of the laser can be increased.